Aqueous dispersed acrylic-epoxy, branched epoxy protective coatings

Abstract

An aqueous protective coating composition particularly useful for can coatings is based on a polymeric binder comprising epoxy crosslinked microgel particles. The microgel polymer is produced by dispersing into water a carboxyl functional, acrylic-epoxy grafted copolymer, and combined with a high molecular weight, branched, multifunctional epoxide resin, followed by coreaction and crosslinking between the carboxyl copolymer and branched epoxide to form stable aqueous dispersed microgel particles. In a preferred aspect of the invention, low molecular weight liquid diepoxide is dispersed into and crosslinked with the carboxyl functional acrylic grafted epoxy copolymer.

I claim: 
     
       1. An aqueous dispersed coating composition containing an aqueous dispersed polymeric binder, the polymeric binder comprising: an aqueous dispersed microgel of a carboxyl functional, acrylic-epoxy grafted copolymer crosslinked with a high molecular weight, branched, multifunctional epoxide resin, where the microgel contains by weight between 50% and 95% said acrylic-epoxy grafted copolymer with the balance being the branched multifunctional epoxide resin; (a) where the acrylic-epoxy grafted copolymer is produced by non-aqueous copolymerization of ethylenically unsaturated monomer in the presence of epoxy resin at temperatures between about 80° and 180° C. in the presence of at least 1% peroxide initiator based on the weight of monomers, the ethylenic monomers having by weight at least 5% carboxyl monomer, to produce the carboxyl functional acrylic-epoxy grafted copolymer having an Acid No. above about 30, where said acrylic-epoxy grafted copolymer is dispersed in water by neutralizing at least part of the carboxyl functionality on said acrylic-epoxy grafted copolymer to preform an aqueous dispersion of carboxyl containing copolymer,   (b) where the high molecular weight, branched multifunctional epoxide resin has a functionality above 2 and a number average molecular weight between about 400 and 6,000 and an epoxy equivalent weight between 400 and 3,000, and where the branched multifunctional epoxide is dispersed into the preformed aqueous dispersed carboxyl copolymer,   (c) where the branched multifunctional epoxide resin is reacted with the aqueous dispersed carboxyl functional acrylic-epoxy grafted copolymer to form aqueous dispersed crosslinked microgel particles having a mean particle size below about 1 micron.     
     
     
       2. The coating composition of claim 1 where the microgel particle size is between about 0.02 and 0.2 microns. 
     
     
       3. The coating composition of claim 1 where the acrylic-epoxy grafted copolymer has an Acid No. between 70 and 300 prior to microgel formation. 
     
     
       4. The coating composition of claim 1 where the branched epoxide resin, prior to crosslinking, has an oxirane content between 0.6% and 4% oxirane oxygen, and a 1,2 epoxy equivalent functionality from about 2.2 to about 5.5 equivalents per mole of branched epoxide resin. 
     
     
       5. The coating composition of claim 1 where the ratio of weight average to number average molecular weight (Mw/Mn) of the branched epoxide resin is above 2.4. 
     
     
       6. The coating composition of claim 1 where the polymeric binder comprises by weight between 50% and 95% said grafted acrylic-epoxy copolymer, between 5% and 50% of said branched multifunctional epoxide resin, and between 4% and 12% of a low molecular weight diepoxide crosslinked with the acrylic-epoxy grafted copolymer. 
     
     
       7. The coating composition of claim 6 where the low molecular weight diepoxide resin, prior to crosslinking is a liquid diepoxide having a number average molecular weight between 200 and 5000, and epoxide equivalent weight between 100 and 2500, where the diepoxide resin is dispersed into the aqueous dispersed carboxyl copolymer and reacted with the carboxyl functional acrylic-epoxy grafted copolymer to form the aqueous dispersed crosslinked microgel particles. 
     
     
       8. In a process for producing an aqueous dispersed coating composition containing an aqueous dispersed, microgel polymeric binder, the process steps comprising: copolymerizing ethylenically unsaturated monomer, including carboxyl functional monomer, by in-situ non-aqueous polymerization in the presence of epoxy resin and in the presence of at least 1% peroxide initiator based on the weight of the monomer copolymerized, at temperatures between about 80° C. and 180° C. to produce a carboxyl functional acrylic grafted epoxy copolymer having an Acid No. above 30;   dispersing the carboxyl functional acrylic grafted epoxy copolymer into water by neutralizing the carboxyl groups on the acrylic rafted epoxy copolymer to provide a prior formed aqueous dispersion of acrylic grafted epoxy copolymer;   dispersing a high molecular weight, branched, multifunctional epoxide resin having functionality above 2 into the prior formed aqueous dispersion of acrylic grafted epoxy copolymer; and   crosslinking the multifunctional branched epoxide resin with the carboxyl functional acrylic grafted epoxy copolymer to produce stabilized, aqueous dispersed, microgel polymer particles having a mean particular size less than 1 micron.   
     
     
       9. The process of claim 8 where the acrylic-epoxy grafted copolymer is first dispersed into water and the multifunctional branched epoxide resin is dispersed into the aqueous dispersion of the said copolymer. 
     
     
       10. The process of claim 8 where a low molecular weight diepoxide resin is additionally dispersed into the aqueous dispersion of the carboxyl functional acrylic-epoxy grafted copolymer, and the diepoxide subsequently is reacted and crosslinked with the carboxyl functional acrylic grafted epoxy copolymer. 
     
     
       11. The process of claim 10 where the resulting microgel polymeric composition comprises by weight between 50% and 94% acrylic grafted epoxy copolymer, between 5% and 50% multifunctional branched epoxide resin, and between 1% and 12% low molecular weight diepoxide resin.